Water and nutrient availability limit plant growth in all but a very few natural ecosystems. They limit yield in most agricultural ecosystems. Plant roots serve important functions such as water and nutrient uptake, anchorage of the plants in the soil and the establishment of biotic interactions at the rhizosphere. Elucidation of the genetic regulation of plant root development and function is therefore the subject of considerable interest in agriculture and ecology.
The root system originates from a primary root that develops during embryogenesis. The primary root produces secondary roots, which in turn produce tertiary roots. All secondary, tertiary, quaternary and further roots are referred to as lateral roots. Many plants, including maize, can also produce shoot borne roots, from consecutive under-ground nodes (crown roots) or above-ground nodes (brace roots). Three major processes affect the overall architecture of the root system. First, cell division at the primary root meristem enables indeterminate growth by adding new cells to the root. Second, lateral root formation increases the exploratory capacity of the root system. Third, root-hair formation increases the total surface of primary and lateral roots (Lopez-Bucio et al., Current Opinion in Plant Biology (2003) 6:280-287). In maize mutants have been isolated that are missing only a subset of root types. In Arabidopsis, mutations in root patterning genes such as SHORTROOT and SCARECROW, which show developmental defects in primary and lateral roots, have been identified (J. E. Malamy, Plant, Cell and Environment (2005) 28: 67-77).
A number of maize mutants affected specifically in root development have been identified (Hochholdinger et al 2004, Annals of Botany 93:359-368). The recessive mutants rtcs and rt1 forms no, or fewer, crown and brace roots, while the primary and lateral roots are not affected. In the recessive mutants des21, lateral seminal roots and root hairs are absent. Root hairs are lacking in the recessive mutant rthl-3. The mutants Irt1 and rum1 are affected before lateral root initiation and mutants slr1 and slr2 are impaired in lateral root elongation. Intrinsic response pathways that determine root system architecture include hormones, cell cycle regulators and regulatory genes. Water stress and nutrient availability belong to the environmental response pathways that determine root system architecture.
U.S. Application No. 2005-57473 filed Feb. 14, 2005 (U.S. Patent Publication No. 2005/223429 A1 published Oct. 6, 2005) concerns the use of Arabidopsis cytokinin oxidase genes to alter cytokinin levels in plants and stimulate root growth.
U.S. Pat. No. 6,344,601 (issued Feb. 5, 2002) concerns the under- or overexpression of profilin in a plant cell to alter plant growth habit, e.g. a reduced root and root hair system, delay in the onset of flowering.
WO2004/US16432 (filed May 21, 2004 (WO2004/106531 published Dec. 9, 2004) concerns the use of methods to manipulate the growth rate and/or yield and/or architecture by over expression of cis-prenyltransferase.
U.S. Application No. 2004/489500 filed Sep. 30, 2004 (U.S. Patent Publication No. 2005/059154 A1 published Mar. 13, 2005) concerns methods to modify cell number, architecture and yield using over expression of the transcription factor E2F in plants.
Activation tagging can be utilized to identify genes with the ability to affect a trait. This approach has been used in the model plant species Arabidopsis thaliana (Weigel et al., 2000, Plant Physiol. 122:1003-1013).
Insertions of transcriptional enhancer elements can dominantly activate and/or elevate the expression of nearby endogenous genes.